New iron-based mixed-polyanion compounds Li(x)Na(4-x)Fe(3)(PO(4))(2)(P(2)O(7)) (x = 0-3) were synthesized, and their crystal structures were determined. The new compounds contained three-dimensional (3D)sodium/lithium paths supported by P(2)O(7) pillars in the crystal. First principles calculations identified the complex 3D paths with their activation barriers and revealed them as fast ionic conductors. The reversible electrode operation was found in both Li and Na cells with capacities of one-electron reaction per Fe atom, 140 and 129 mAh g(-1), respectively. The redox potential of each phase was ∼3.4 V (vs Li) for the Li-ion cell and ∼3.2 V (vs Na) for the Na-ion cell. The properties of high power, small volume change, and high thermal stability were also recognized, presenting this new compound as a potential competitor to other iron-based electrodes such as Li(2)FeP(2)O(7), Li(2)FePO(4)F, and LiFePO(4).

The development of efficient and stable water oxidation catalysts is necessary for the realization of practically viable water-splitting systems. Although extensive studies have focused on the metal-oxide catalysts, the effect of metal coordination on the catalytic ability remains still elusive. Here we select four cobalt-based phosphate catalysts with various cobalt- and phosphate-group coordination as a platform to better understand the catalytic activity of cobalt-based materials. Although they exhibit various catalytic activities and stabilities during water oxidation, Na2CoP2O7 with distorted cobalt tetrahedral geometry shows high activity comparable to that of amorphous cobalt phosphate under neutral conditions, along with high structural stability. First-principles calculations suggest that the surface reorganization by the pyrophosphate ligand induces a highly distorted tetrahedral geometry, where water molecules can favourably bind, resulting in a low overpotential (∼0.42 eV). Our findings emphasize the importance of local cobalt coordination in the catalysis and suggest the possible effect of polyanions on the water oxidation chemistry.

Battery chemistry based on earth-abundant elements has great potential for the development of cost-effective, large-scale energy storage systems. Herein, we report, for the first time, that maricite NaFePO 4 can function as an excellent cathode material for Na ion batteries, an unexpected result since it has been regarded as an electrochemically inactive electrode for rechargeable batteries. Our investigation of the Na re-(de)intercalation mechanism reveals that all Na ions can be deintercalated from the nano-sized maricite NaFePO 4 with simultaneous transformation into amorphous FePO 4 . Our quantum mechanics calculations show that the underlying reason for the remarkable electrochemical activity of NaFePO 4 is the significantly enhanced Na mobility in the transformed phase, which is $one fourth of the hopping activation barrier. Maricite NaFePO 4 , fully sodiated amorphous FePO 4 , delivered a capacity of 142 mA h g À1 (92% of the theoretical value) at the first cycle, and showed outstanding cyclability with a negligible capacity fade after 200 cycles (95% retention of the initial cycle).The demand for large-scale energy storage systems (EESs) has prompted considerable effort in the development of new types of batteries with cost-effective and sustainable properties. While the high cost of current Li ion batteries (LIBs) remains one of the major hurdles towards large-scale energy storage applications, 1-12 battery chemistry based on earth-abundant elements offers a feasible solution. Recently, Na ion batteries (NIBs) have been considered as a promising alternative to LIBs since the underlying electrochemical reaction is similar to that of LIBs, but is based on the unlimited resources of Na from seawater. [13][14][15][16][17][18][19][20] The use of redox chemistry using earth abundant transition metals would provide the optimal combination with Na electrochemistry further highlighting the advantage of NIBs.In recent years, considerable research has been carried out on Fe-based electrode materials for use in NIBs. Broader contextWe report, for the rst time, that maricite NaFePO 4 can function as an excellent cathode material for Na ion batteries, an unexpected result since it has been regarded as an electrochemically inactive electrode for rechargeable batteries. Our investigation of the Na re-(de)intercalation mechanism reveals that all Na ions can be deintercalated from the nanosized maricite NaFePO 4 with simultaneous transformation into amorphous FePO 4 . Our quantum mechanics calculations show that the underlying reason for the remarkable electrochemical activity of NaFePO 4 is the signicantly enhanced Na mobility in the transformed phase, which is $one fourth of the hopping activation barrier. Maricite NaFePO 4 , fully sodiated amorphous FePO 4 , delivered a capacity of 142 mA h g À1 (92% of the theoretical value) at the rst cycle, and showed outstanding cyclability with a negligible capacity fade aer 200 cycles (95% retention of the initial cycle).540 | Energy Environ. Sci., 2015, 8, 540-545This jour...

Compounds with a mixed polyanion framework have recently gained attention as a new class of compounds for material exploration. The potential tunability of the structure by using various combinations of polyanions can potentially lead to a novel cathode. However, the redox reaction in complex structures often involves complex structural evolutions during the electrochemical reaction, which require careful analysis. We investigated the electrochemical mechanism of Na x Fe3(PO4)2(P2O7) (1 ≤ x ≤ 4), which was recently proposed as a promising mixed-polyanion cathode for Na rechargeable batteries, using first principles calculations and experiments. We discovered that the de/sodiation of the Na x Fe3(PO4)2(P2O7) electrode occurs via a one-phase reaction with a reversible Fe2+/Fe3+ redox reaction and accompanies an exceptionally small volumetric change of less than 4%. Na ion intercalation usually induces a large volumetric change in conventional systems; therefore, this small volume change is unusual and was attributed to the open framework of polyanion compounds with P2O7 dimers that are capable of rotating and distorting to accommodate the structural change. Structural robustness was further observed at even highly charged states at temperatures as high as 530 °C from in situ X-ray diffraction (XRD) and differential scanning calorimetry (DSC). We believe that the improved understanding of the electrochemical mechanism provided here will expedite the optimization of the new Na4Fe3(PO4)2(P2O7) electrode for Na rechargeable batteries.